Optical sensor

ABSTRACT

An optical sensor ( 10 ) comprises an optical cavity defined by a dielectric body and responsive to one or more physical environmental conditions, and a waveguide ( 70 ) having a terminal end spaced apart from the optical cavity such that light is optically coupled from the terminal end of the waveguide ( 70 ) to the optical cavity. The waveguide ( 70 ) is arranged such that, in use, it is maintained at a first temperature that would not damage the optical coupling to the optical cavity when the dielectric body is maintained at a second temperature sufficient to damage the optical coupling to the optical cavity.

FIELD OF THE INVENTION

The present invention relates to an optical sensor and more specificallyto an optical sensor for measuring pressure and/or temperature.

BACKGROUND OF THE INVENTION

WO 99/60341 describes an optical sensor, fabricated using conventionalmicromachining techniques, for measuring changes in temperature andpressure inside a combustion engine. The sensor comprises a slab ofsilicon having a recess defined in one surface by etching. Amicro-capillary, having a silica fibre fixed inside, is adhered to thesilicon slab so as to close the recess. The face of the silica fibre andthe inner surface of the recess directly opposite the fibre serve todefine a Fabry-Perot cavity. Light incident along the silica fibre isreflected within the Fabry-Perot cavity and guided back along the silicafibre. The reflected light creates interference fringes whosecharacteristics are determined by the length of the Fabry-Perot cavity.Changes in the external pressure cause the wall of the silicon slabdirectly opposite the fibre to deflect, causing a change in the lengthof the Fabry-Perot cavity. This in turn creates a change in thecharacteristics of the interference fringes thus registering a change inpressure. The sensor may also be used to sense changes in temperature byemploying a suitably thick slab of silicon. Changes in temperature causethe slab to expand or contract, which in turn results in a similarexpansion or contraction of the Fabry-Perot cavity.

Whilst the silicon sensor may be used for many applications, the sensoris unsuitable for environments that are at elevated temperatures. Inparticular, the maximum temperature at which the silicon sensor canoperate is around 450° C. Above this temperature, the elastic propertiesof silicon become unstable making any measurements unreliable.

WO 2005/098385 describes a sapphire optical sensor sensitive to bothpressure and temperature. A waveguide formed from an optical fibre,hollow ceramic rod or metal tube is used to interrogate the opticalsensor. The waveguide is bonded directly to the optical sensor using oneof a number of bonding techniques. In one embodiment a sapphire opticalfibre is fusion bonded to the optical sensor at temperatures between600° C. and 1500° C.

In an alternative embodiment described in WO 2005/098385, the waveguideis spaced from the optical sensor by a short distance of around 3-100μm. However, this sensor is not suitable for use at elevatedtemperatures as the fusion bond between the waveguide and the opticalsensor will weaken and may fail between 600° C. and 1500° C.

US 2007/0013914 describes an optical fibre sensor formed by bonding asapphire membrane to the end of a capillary tube and bonding an opticalfibre within the capillary tube so that the end of the optical fibre andthe near (to the optical fibre) surface of the sapphire membrane definea first optical cavity. The optical fibre may be bonded with epoxy orlaser welded to the capillary tube.

A second optical cavity is defined by the near and far surfaces of thesapphire membrane and is used to obtain a compensating temperaturemeasurement. However, due to thermal mismatch between the optical fibreand capillary tube this sensor is not suitable for use at hightemperatures.

These prior art devices are not suitable for use at elevatedtemperatures or to be cycled repeatedly from low to high temperatureswithout structural damage due to thermal mismatch. Therefore, there isrequired an optical sensor that overcomes these problems.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is providedan optical sensor comprising an optical cavity defined by a dielectricbody. The optical cavity is responsive to one or more physicalenvironmental conditions such as for instance, temperature and pressure.A waveguide having a terminal end is separated from the optical cavityand arranged so that light is coupled from the waveguide to the opticalcavity. In use the waveguide is maintained at a lower temperature thanthat of the dielectric body so that the optical cavity may be responsiveto environments at higher temperatures than would otherwise damage thewaveguide.

Preferably, the optical cavity may be a Fabry-Perot cavity interrogatedby a broadband source of radiation emitted and collected by thewaveguide.

According to a second aspect of the present invention the optical sensorfurther comprises a temperature reduction means for maintaining thewaveguide proximal to the terminal end at or below the firsttemperature. In other words, the temperature reduction means allows theoptical sensor to operate in environments at temperatures that wouldotherwise damage the optical coupling between the waveguide and opticalcavity. This damage may for instance, soften or melt the waveguide orother optical components or mechanical joints. This allows the waveguideto be kept close enough to the optical cavity such that misalignment ofa beam emitted from the waveguide on to the optical cavity is reduced.This allows the optical sensor to measure environments at highertemperatures or higher temperature cycling than would otherwise damagethe waveguide or optical coupling.

Preferably, the temperature reduction means may be arranged to draw heataway from the waveguide.

Preferably, the optical sensor may further comprise a housing having aproximal end and a distal end and arranged to support the dielectricbody at the proximal end and support the waveguide at the distal end.

Optionally, the temperature reduction means may comprise a tubesurrounding the waveguide. When placed in a temperature gradient throughfor instance, an engine casing, the distal end may be cooler than theproximal end and any excess heat is drawn away from the waveguide by thetemperature reduction means. The tube may have any suitable crosssection including circular and may fit loosely or snugly around thewaveguide.

Preferably, the temperature reduction means may be formed of metal, suchas for instance copper or another suitable heat conductive metal.

In accordance with a third aspect of the present invention the terminalend of the waveguide is far enough from the optical cavity so that thewaveguide is maintained at a temperature that does not damage it in usewith the sensor operating a temperature at the dielectric body end thatwould damage the optical coupling. When placed across a temperaturegradient the optical cavity may be placed at the hotter end and theterminal end of the waveguide may be located towards the cooler end ofthe sensor in use.

Preferably, the distance is greater than 1 mm. More preferably thedistance is greater than 10 mm. More preferably the distance is equal toor greater than 25 mm.

Advantageously, the optical sensor may further comprise an opticalalignment mechanism for providing alignment between the waveguide andthe optical cavity. This allows the waveguide to be separated furtherfrom the optical cavity (at the hotter end of the sensor) and stillmaintain sufficient alignment of a beam emitted from the waveguide andincident on the optical cavity.

Advantageously, the alignment mechanism may comprise a ball joint havinga ball arranged to support the waveguide and a socket arranged toadjustably support the ball.

Preferably, the ball joint may be arranged within a housing. With a beamemitted and collected by the waveguide the ball joint may be manipulatedto maximise the received signal. At a maximised position the ball maythen be fixed in position.

Preferably, the ball may be fixed in position relative to the socket bya fixture of for example, a weld.

Optionally, the waveguide and optical cavity may be aligned by mountingthe waveguide off axis within a mount. The waveguide may be rotateduntil the signal is maximised and fixed in position when a maximumsignal is achieved.

Advantageously, the optical sensor may further comprise a collimator forcollimating the light emitted from the waveguide. This increases theuseable amount of light emitted from the waveguide and further allows anincrease in distance between the optical sensor and the waveguide. Acollimated beam allows the separation between waveguide and opticalcavity to be greater than 0.5 mm. Without a collimator at greaterseparations alignment becomes very difficult.

Preferably, the collimator may comprise a lens attached to the terminalend of the waveguide by an attachment. Attaching a lens to the end ofthe waveguide allows the alignment mechanism to align the lens andwaveguide simultaneously.

Optionally, the attachment may be a fusion bond between the waveguideand the lens. Fusing the waveguide to the lens provides a strongerattachment and allows the fibre to be physically supported at a singlepoint within the optical sensor, i.e. the fusion bond between theterminal end of the waveguide and the lens. Fusion bonding also providesgreater thermal resilience and may reduce optical losses at theinterface between waveguide and lens.

Optionally, the lens may comprise a first surface including a curvedportion surrounded by a planar portion.

Advantageously, the lens may further comprise a planar second surfaceparallel with the planar portion of the first surface.

Optionally, the curved portion or portions may be formed bymicro-machining. The lens may also take the form of a fresnel lens ordiffractive lens. These or other lens forms may be made by etching.

Optionally, the lens may be formed from sapphire or silica. Bondingbetween a silica lens and a silica fibre (where used) is thereforesimplified and more straightforward.

Optionally, the optical sensor may further comprise a spacer extendingfrom the dielectric body towards the terminal end of the waveguide andsurround the light emitted from the waveguide. The dielectric body maybe bonded to the spacer, which may be a hollow tube or a solid rodtransparent to the beam emitted from the waveguide. A hollow tube mayreduce heat conduction from the hotter end of the optical sensor to thecooler end. A solid rod may reduce complexity of the optical sensor andreduce residual stresses. The bonding technique used may preferablywithstand the high temperatures encountered. Suitable bonding techniquesincluded for instance, thermocompression bonding, eutectic flux bondingusing materials such as Yttria (see U.S. Pat. No. 6,012,303), laserwelding or laser assisted bonding. A thermal compression seal may reducestresses on the spacer during temperature cycling of the optical sensor.

Preferably, any stresses in the spacer may be removed before and afterbonding.

Optionally, the spacer may be sapphire, silica, magnesium oxide, MgAlO,alumina, zirconia or other similar materials. Preferably, the spacer anddielectric body may have very similar coefficients of thermal expansion(CTE) to reduce additional stresses placed on each component when thesensor is heated in use. Therefore, it is advantageous to form thespacer and dielectric body out of the same material, e.g.sapphire-sapphire or MgO—MgO, for instance. The spacer may be made froma polycrystalline phase material (e.g. alumina) but should have asimilar CTE to that of the material used for the dielectric body.

Preferably, the spacer may be bonded to a mount. The mount may beintegral with a housing.

Advantageously, the mount may be kovar or similar alloy capable of useat high temperatures without significant thermally induced changes.

Optionally, the spacer may be bonded to the mount by oxide bonding.Oxide bonding may provide, a hermetic seal.

Preferably, a thermal compression seal may be arranged to separate thespacer from a mount at an end of the spacer distal from the dielectricbody.

Advantageously, the thermal compression seal may be a low-creep washersuch for instance, copper or platinum. Advantageously, the washer may beformed from an oxidation resistant ductile material or metal such asplatinum or gold, for instance.

Preferably, the washer may be grain-stabilised to further reduce creep.Such washers may apply sufficient compression over a wide temperaturerange and be able to absorb stresses due to thermal mismatch, which mayotherwise shatter the spacer. In this way the spacer may be held using alow modulus material in compression over a large temperature range andcycle.

Preferably, where a sapphire spacer is used, the axis along the sapphirespacer may be collinear with the sapphire C-axis, i.e. its axis of zerobirefringence. An advantage of the use of the C-axis is that the opticaxis of sapphire coincides with the C-axis so the effect ofbirefringence is reduced. Consideration of crystal lattice orientationimproves thermal matching. Therefore, it is preferable that the axis ofthe sapphire spacer matches that of the sensor pill. Other orientationsmay be possible from a mechanical point of view so long as birefringenceis considered.

Advantageously, the spacer may be thermally and/or mechanically isolatedfrom the housing. This may reduce vibration that could interfere withthe varying pressure signal (sound).

Optionally, the spacer may further comprise a spacing protrusionarranged to separate the spacer from the housing. This allows astructure to be formed that has a simplified or single mode of vibrationdue to a single point of contact. Preferably, this mode may be chosen ordesigned to be away from any particular frequencies of interest for thedevice (e.g. typical frequencies generated by an engine beingmonitored). The natural or resonant frequency of this vibration mode maythen be damped without affecting the desired sensitivity of the deviceover a particular frequency range.

Optionally, the spacing protrusion may be an annular protrusion.

Preferably, the spacing protrusion may be integral with the spacer.

Advantageously, the spacing protrusion may be separated from theproximal end of the housing. This may further reduce heat conduction tothe fibre or lens arrangement.

Advantageously, the waveguide is maintainable below about 700° C. andthe dielectric body may be maintainable above about 1000° C. Suitableapplications for this optical sensor include sampling pressure and/ortemperature within a gas turbine or jet engine.

Preferably, the optical cavity in the dielectric body may define at oneend a membrane deflectable in response to changes in external pressure.Absolute and instantaneous pressure may be monitored by the opticalsensor.

Optionally, the membrane may be concave in the direction facing theterminal end of the waveguide. In this configuration the optical sensormay be less sensitive to misalignment of the beam emitted from thewaveguide.

Optionally, the dielectric body may further comprise one or morepressure equalising channels communicating between the interior andexterior of the optical cavity. This reduces the pressure differenceacross the membrane allowing a thinner and more deflectable membrane.Such a sensor may also be more sensitive to small changes in pressurewaves such as sound waves.

Preferably, the waveguide may be a single mode waveguide and thedielectric body may be sapphire or magnesium oxide. As mentioned above,the material of the dielectric body should match the CTE of the spacer.

Preferably, the waveguide is formed from sapphire or silica. Similartransparent high temperature ceramics may also be used.

According to a third aspect of the present invention there is provided agas turbine engine comprising an engine casing having an inner surface,the inner surface enclosing a hot gas space having an elevated internaloperating temperature, and an optical sensor disposed through anaperture in the inner surface. The optical sensor comprises an opticalcavity defined by a dielectric body and responsive to one or morephysical environmental conditions such as for instance, temperature andpressure, and a waveguide with a terminal end optically coupled to theoptical cavity, wherein the dielectric body is exposed to the hot gasspace. The dielectric body may communicate with the hot gas space by forinstance, extending through the inner surface in the engine casing intothe hot gas space. Hot gases from within the engine may alternatively bedirected to the dielectric body, which may be place flush with the innersurface or placed within the engine casing.

The optical sensor may also be used within other types of engines suchas, for instance within the combustion chamber of internal combustionengines.

Preferably, the optical sensor may be arranged such that the waveguideis maintained, in use, at a lower temperature than the elevated internaloperating temperature by temperature reduction means. The internaloperating temperature exposed to the dielectric body would damage thewaveguide or optical coupling between the waveguide and optical cavity.However, the temperature reduction means maintains the waveguide at alower temperature to avoid this damage, whilst at the same time thedielectric body (which can withstand such elevated temperatures) is incontact with the hot gas space.

Advantageously, the temperature reduction means may be arranged to drawheat away from the waveguide.

The optical sensor of the gas turbine engine according to the thirdaspect of the present invention may comprise any or all of the featuresof the optical sensor described above with regards to the other aspectsof the present invention.

Optionally, a central portion of the membrane may be thickened to reduceoptical distortion of reflected light from this thickened centralportion. This reduces fringe fading as light may be reflected from arelatively flatter surface.

Preferably, the waveguide may be an optical fibre.

Optionally, the optical fibre may attach to a planar waveguide at itsterminal end. The planar waveguide may then shine light onto the opticalcavity.

Optionally, the collimator described above may be in the form of a taperor similar structure on the waveguide, the planar waveguide or theoptical fibre described above.

BRIEF DESCRIPTION OF THE FIGURES

The present invention may be put into practice in a number of ways andan embodiment will now be described by way of example only and withreference to the accompanying drawings, in which:

FIG. 1 shows a cross-sectional view of an optical sensor including thetemperature and pressure sensing optical cavities, according to a firstembodiment of the present invention, given by way of example only;

FIG. 2 shows a schematic cross-sectional view of an optical sensoraccording to a second embodiment of the present invention, given by wayof example only;

FIG. 3 shows a cross-sectional view of an optical sensor according to athird embodiment of the present invention;

FIG. 4 shows a schematic diagram of the optical cavities of FIG. 1;

FIG. 5 shows a schematic diagram of an interferometer including a phasemodulator and light source used to illuminate and detect light from theoptical sensors of FIGS. 1-3;

FIG. 5 a shows a schematic diagram of an alternative interferometer tothat shown in FIG. 5;

FIG. 5 b shows a schematic diagram of further alternative interferometerto that shown in FIG. 5, including two light sources;

FIG. 5 c shows a schematic diagram of an example multiple interferometerinterrogator used to illuminate and detect light from the opticalsensors of FIGS. 1-3;

FIG. 6 shows a graph of received light intensity versus the optical pathdifference induced in the phase modulator of FIG. 5 for an exampleoptical sensor according to one embodiment of the present invention;

FIG. 7 shows a side view of a lens and optic fibre mount used within afourth embodiment of the present invention;

FIG. 8 shows a perspective view of the lens mount of FIG. 7;

FIG. 9A shows a cross-sectional view of a lens and optic fibrearrangement used within a fifth embodiment of the present invention withhidden features shown in dotted lines;

FIG. 9B shows a cross-sectional view along line C-C of FIG. 9A of thelens and optic fibre arrangement of FIG. 9A;

FIG. 9C shows a partial cross-sectional view of the lens of FIGS. 9A and9B;

FIG. 10 shows a perspective view of a lens having a curved portion usedwithin a sixth embodiment of the present invention;

FIG. 11 shows a magnified view of the curved portion of FIG. 10;

FIG. 12 shows a perspective view of a mount supporting the lens of FIGS.9A-11;

FIG. 13 shows a schematic diagram of an example optical filterarrangement used to generate the two light sources shown in FIG. 5 b;

FIG. 14 a shows an end view of an alternative embodiment to the exampleoptical sensor of FIG. 1;

FIG. 14 b shows a side view of the optical sensor of FIG. 14 a;

FIG. 14 c shows a cross-sectional view the optical sensor of FIG. 14 a;and

FIG. 14 d shows a perspective view the optical sensor of FIG. 14 a.

It should be noted that the figures are illustrated for simplicity andare not necessarily drawn to scale.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a cross-sectional view of an optical sensor according to afirst embodiment of the present invention. In use, a waveguide deliversa beam of light to a sensor element 20 formed from a dielectric materialand defining an optical cavity in the form of a Fabry-Perot cavity. Inthe preferred embodiment the waveguide is an optical fibre 70 but anysuitable waveguide may be used. The sensor element 20 may define one ormore optical cavities that may cause interference to occur in beamsreflecting off various surfaces within the sensor element 20. Thereflected beams are collected by the same optical fibre 70 used toilluminate the sensor element 20 and these reflected beams areinterrogated by an interferometer (not shown in this Figure).

Pressure and/or temperature changes alter the physical configuration ofthe sensor element 20 leading to changes in the interference patterngenerated and detected and these changes in interference patterns aredecoded to indicate the pressure and temperature at the sensor elementsite. The face of the sensor element 20 facing away from the fibre 70 isformed from a membrane that is deflectable by changes in pressure. Oncethe membrane is deflected the dimensions, of the optical cavity withinsensor element 20 change leading to a change in the resultantinterference pattern. A thicker layer of the material forming the sensorelement 20 faces the optical fibre 70 and thermal expansion of thisthicker layer results in a change in a further optical cavity dimensionand again this leads to a change in the interference pattern which isdecoded by the detection interferometer (not shown in this Figure).

The sensor element 20 is made from sapphire and bonded to a sapphirespacer 30, which may be a solid rod or a hollow tube. As the sensorelement 20 sits at the hotter end of the optical sensor 10 in use thisbond is preferably strong and may be formed by thermocompression, laserwelding or laser assisted bonding or any other suitable bondingtechnique. Preferably this seal is hermetic to reduce the risk of hotgasses at the sensing end of the optical sensor from penetrating thedevice.

A cap 90 may be placed over the sensor element 20 to protect it or toprotect the interior of the tested environment, such as a gas turbineengine or jet engine, for instance. In an alternative embodiment gauzemay be placed over the cap to increase protection from foreign bodies.

A mount 40 supports the spacer and may be bonded to the spacer by anoxide seal. To prevent damaging stresses from occurring a compressionwasher 35 separates the end of the spacer facing the optical fibre 70from the mount 40. Preferably the compression washer is formed from amaterial to minimise creep, such as copper or platinum and grainstabilised to further reduce creep when thermal cycling occurs. The cap90 may be welded to the mount 40.

An outer sleeve 80 protects the optical sensor and provides strainrelief at the emerging end of the optical fibre 70. The end of theoptical fibre facing the sensor element 20 incorporates a collimator.The collimator may be a lens 60 fusion welded to the terminal end of theoptical fibre 70. The lens is secured by a collet and the collet issecured within a ball 50 within a socket defined in a mount 40. In thisway the optical fibre 70 may be secured at a single point, i.e. thefusion weld to the lens 60, which may be the only place that may stressthe optical fibre 70. The ball and socket arrangement allows alignmentof the fibre 70 and lens 60 arrangement to be undertaken during analignment procedure. The lens 60 and optical fibre 70 arrangement may bealigned to the sensor element 20 by illuminating the sensor element 20and monitoring the output until the signal is maximised indicating thatthe optical fibre 70 and sensor element 20 are aligned. Once alignmentis achieved the ball 50 may be fixed relative to the mount 40 by asuitable permanent technique such as e-beam or laser welding throughdedicated ports in the mount 40, for instance. Alignment of the beamemitted from the fibre 70 to the sensor element 20 within 0.1° isdesirable.

Alternatively, the optical fibre may be located off axis and aligned byrotating the optical fibre 70 until the signal is maximised.

A shield (not shown in this Figure) protects the free end of the opticalfibre (70).

The sensor element 20 may be preferably manufactured from a refractorymaterial such as, sapphire or magnesium oxide. These materials aretransparent in the visible and infrared wavelength ranges, typically atleast around 1300 nm and 1550 nm but other wavelengths and wavelengthranges are suitable. The sensor element 20 may be produced by bondingslices of material together. The cavity may be defined by an etchingtechnique such as dry etching or chemical etching. A slice may bethinned to allow a significant pressure response forming a pressuresensitive membrane. The optical fibre 70 may be a single mode fibre.

As described above, a single or multiple optical cavities may be formedin the sensor element 20. If a single (pressure only sensing) opticalcavity is required, then the surfaces of the optical sensor 20 notforming the cavity may be wedged to frustrate any further cavities. Thisprovides a simplified device as only one optical cavity needs to beinterrogated. A suitable wedge angle for the front surface of the sensorelement 20 may be 5°, for instance. Such wedging also avoids coupling ofunwanted reflections where contaminants such as soot deposits, forinstance, change the reflectivity of exposed optical surfaces.

FIG. 2 shows a schematic diagram of a second embodiment of the presentinvention. This is a simplified diagram and not all components areshown. In this embodiment the optical fibre 70 is held close to theoptical sensor 20 towards the hotter end of the outer sleeve 80. In thisalternative configuration, the alignment tolerance between the beamemitted from the optical fibre 70 and the optical cavity 200 is reducedas they are spatially nearer to each other. The minimum separationdepends on thermal considerations, i.e. how hot the optical sensor 20 isin use and how efficiently the optical fibre 70 may be cooled.Evacuating the interior of the device further reduces heat transfer tothe optical fibre 70. For instance, this configuration may allow theoptical fibre 70 and optical sensor 20 to be less than or equal to about0.5 mm apart therefore allowing satisfactory coupling without requiringa collimator or lens. Furthermore, this embodiment does not require sucha sophisticated alignment mechanism. Instead, the optical fibre 70 isheld within a supporting tube 210, which acts as a heat sink to drawheat away from the optical fibre and to keep the optical fibre 70 withinits optimal temperature range. The heat sink tube 210 is attached to thecooler end of the outer sleeve 80. In this embodiment no collimatinglens is required. Nevertheless, a small separation between the opticalelement 20 and the terminal end of the optical fibre 70 reduces thermalcontact between those components.

Not shown in FIG. 2 is sleeving used to protect and stress relieve theoptical fibre 70 as it exits the back of the outer sleeve 80. The methodof sleeving may be conventional but the material may preferably becapable of withstanding 700° C. It will be appreciated that in oneexample embodiment of the present invention the sensor design may betailored to fit in a hole in the side of a gas turbine of internalcombustion engine, so that the sensor may be flush with the inner wallof the engine and is therefore capable of measuring for examplepressure. The back end (with the optical fibre 70 exiting the opticalsensor) may be flush with the outer wall of the engine and so thetemperature gradient along the package may largely be dictated by theheat flux in the engine wall, and the outer engine temperature maydictate the back end temperature of the optical sensor.

In an alternative configuration the optical sensor may be held at itsrear end (optical fibre 70 end) so the optical sensor may reach into ahot environment (say protruding into the combustion area of an engine).In this configuration the package may have thinner walls/longer lengthto allow for lower back end temperatures.

FIG. 3 shows a cross-sectional view of a third embodiment of the presentinvention. Similar elements have been given the same reference numeralas the previous embodiments. The optical sensor 10′ is similar to theoptical sensor 10 of FIG. 1. Spacer tube 130 is partially tapered ateach end with the angles of taper at each end being different. Spacer135 is a hollow tube arranged so that the beam emitted from the opticalfibre 70 passes through the hollow section of the tube. The end of thespacer tube 130 that the optical element 20 is attached to is taperedwith taper 135. The other end of the spacer tube 130 that faces theterminal end of the optical fibre 70 is tapered with taper 137. Theangle of taper 135 to the axis of spacer tube 130 is smaller than thecorresponding angle for taper 137.

A machined mount 40′ receives the spacer tube 130 and a dish shapedcompression washer 35′ separates the end of spacer tube 130 from themount 40′ reducing the stresses due to thermal expansion occurringwithin the spacer tube 130. As in the first embodiment the spacer tube130 may be made from a refractory material such as sapphire or magnesiumoxide, for instance. Where sapphire is used, the axis along the lengthof the spacer tube 130 may correspond with the C-axis of sapphire.

The spacer tube 130 is shaped so that taper 135 holds it undercompression and forms an oxide seal between the aluminium oxide in thesapphire (or magnesium oxide) and the inner surface of the mount 40′.Taper 137 and a corresponding taper in the mount 40′ have different coneangles to ensure alignment under compression. The angle of taper 135 ischosen as a compromise; if it were a bigger taper such as for instance,up to 45° there may be a high stress concentration at the sensor elementbonding area. A smaller angle allows self locking and turns high axialloading into smaller radial loading, which is more desirable to providecontinuous compression thereby reducing the maximum stress seen by thespacer tube 130. The main pressure seal is provided by the compressionwasher 35′. The angle of taper 137 rear angle is about 45° as a gentletaper here reduces the overall length.

The outer diameter of the spacer tube 130 provides a linear alignmentguide with the inner diameter of a bore through the mount 40′. Thus thetwo tapers 135 and 137 may work against each other to hold the spacertube 130 in place and more accurately aligned. The collimation lengthcould be increased further reducing the temperature of operation of thismain seal but increasing the alignment requirements.

One advantage of this design is that there is a free space path betweenthe optical fibre 70, which may not survive the high temperatures thatthe sensor element 20 may withstand. A suitable distance between theterminal end of the optical fibre 70 (or its collimator) and the sensorelement 20 may be around 50-100 mm. This may allow the sensor to be usedin harsh environments such as within gas turbine engines where thetemperature falls rapidly away from the combustion zone to temperaturesof the order of 600° C., which optical fibres 70 may withstand.

A cap 90′ secures the spacer tube 130 and optical element 20 assembly tothe mount 40′. A sleeve weld 125 secures the cap 90′ to the mount 40′.

The cap 90′ also prevents any internal components of the optical sensor10′ from entering the environment to be sensed such as, for instance, agas turbine or jet engine or for debris to enter or damage the sensor. Abore through the cap 90′ allows the optical element 20 to communicatewith the environment to be sensed. The inner surface of this bore may beangled by around less than 1° to the normal of the axis of the opticalsensor 10′ to reduce reflections from the cap being fed back into thesensor.

In this embodiment the sleeve is welded by weld 145 to the mount 40′. Aswith the first embodiment a ball joint alignment mechanism allows theoptical fibre 70 and lens 60 assembly to be aligned with the opticalcavity 200. A collet 165 secures the lens 60 within the ball 50.

The collet 165 is a split collet and the lens 60 is press fitted intothe collet 165, which is itself a press fit into the ball 50. The ball50 is preferably made of Kovar and the collet 165 of a soft metal suchas copper so thermal expansion doesn't allow the lens 60 to becomeloose, as the initial loading allows for that. In practice this requirescareful material selection and preparation to avoid over compressing thelens 60 or it becoming loose due to stressing the collet 165 beyond itsyield point. In an alternative embodiment, these problems are avoided bypressing the lens 60 into a Kovar collet at a temperature of severalhundred degrees C. higher than the temperature the structure will see inuse. Then when cool the collet and lens may be press fitted into theball. The inner bore of the collet 165 may be oxidised to give a bettersurface for the lens 60 to bear against, and to assist retention by theformation of a chemical bond between the silica of the lens 60 and thecollet oxide.

FIG. 4 shows a schematic cross-sectional view through the opticalelement 20. The optical element 20 comprises two parts, slab 400 andslab 420. Slab 400 is etched to form a pit 200 leaving a thin membrane410. Slab 420 is not etched and is a uniform disc although other shapesare suitable. A chlorine-based chemistry, preferably with reactive ionetching, may be used to form the pit.

Pit 200 is defined by depth d1. The thickness of the membrane 410 is d2and the thickness of slab 420 is d3.

Several optical cavities may be defined within sensor element 20 by thevarious surfaces. Each of these optical cavities may be a Fabry-Perotcavity. The pit 200 may be 1 mm in diameter and the diameter of eachslab may be 4 mm. However, other dimensions may be suitable. d3 maybe >200 μm, d2 may be of the order of 100 μm and d1 may be between 3 and50 μm. When a pressure differential is applied across membrane 410 thismembrane deflects thereby changing dimension d1. The faces of slab 420provide an additional optical cavity whose dimensions change as thetemperature changes due to thermal expansion. Typically, the change indimension for a 100 μm slab of sapphire is approximately 8×10⁻¹⁰ m per °C.

In an alternate embodiment both slabs may be etched to form pits, sothat when the slabs are bonded together the pits may face each other toform a single cavity.

After the two slabs have been bonded together slab 400 may be polishedto reduce further the thickness d2 of the membrane. Typically forpressure differences of a few bar upwards membrane thicknesses of 50-100μm may give a deflection of 0.3 μm for a diameter of 1 mm. With theseexample dimensions the membrane remains substantially flat, which limitsthe stress in the diaphragm and therefore limits the possibility of longterm creep. These dimensions also provide membrane deflections that areless than a wavelength of near infrared light typical for availabletelecom grade components (1300 nm or 1550 nm) in order to furthersimplify interrogation.

In an alternative embodiment membrane 410 has a thicker boss at thecentre which does not deform as much as the remaining part of themembrane and so will remain substantially flat during the main movement.This minimises fringe fading during interrogation. A further alternativeembodiment uses a membrane that is concave in the direction facing theoptical fibre 70. This alternative embodiment reduces the effect ofangular drift and alignment errors in the fibre-lens assembly. Theradius of concave curvature may be approximately the same as thedistance between lens 60 and membrane 410. Such profiling also assistswith the focussing and collimation of the incident beam back to thefibre 70.

The sensor element 20 may be interrogated by illuminating it with light.Various sources of light may be used. Light sources include lasers andsuperluminescent laser diodes (SLD), for example. A photodetectordetects the light reflected from the sensor element 20. In this way thesensor element 20, light source and photodetector may be arranged toform an interferometer. The intensity at the photodetector will varydepending on the wavelength of the light source and the lengths of theoptical cavities in the sensor element 20. In other words, interferencefringes caused by the optical cavities, may be detected by thephotodetector. With a fixed wavelength light source, changes in thelength of the optical cavities may be measured by correlating theintensity of the detected light with a particular portion (from maximumto minimum) of an interference fringe (assuming that the change inlength resulted in a change due to less than one fringe). As the lengthin a particular optical cavity changes a sinusoidal variation inintensity will be measured at the photodetector, assuming no opticallosses occur. For instance, a lookup table of intensities may begenerated against cavity length in order to generate the requiredcorrelation so the measured intensity relates to one particular cavitylength.

However, if a laser were used as the light source (having a coherencelength greater than the largest optical cavity length) it may bedifficult to differentiate between the fringes caused by each opticalcavity, d1, d2 and d3. Furthermore, other parasitic cavities may exist(e.g. between the terminal end of the optical fibre 70 and the back faceof the sensor element 20) that may also contribute further unwantedfringe patterns. The use of an SLD may remove various fringes from theoutput as the coherence length of the light produced by the SLD may bechosen to be low enough to discriminate against larger cavity lengths;light interfering within larger optical cavities will not be coherentand so will not caused fringes. For instance, if d1 is chosen to have asmaller optical cavity length than d2 and d3, and the coherence lengthof the SLD was less than d2 and d3, (but greater than d1) d1 will be theonly optical cavity to give rise to interference fringes.

Coherence length in SLDs is typically proportional to output wavelengthbandwidth. It may be difficult to obtain SLDs with sufficiently lowcoherence length to be able to discriminate only the smallest opticalcavity length, d1, within the sensor element 20. The coherence length ofthe incident light may then be deliberately reduced by introducing asecond SLD light source having a nominally similar wavelength (usuallywithin about 50 nm) to the first SLD providing the required coherencelength.

In an alternative embodiment two lasers, each having differentwavelengths, may be used as the light source. This gives rise to twoseparate interference fringes for each optical cavity. Instead of usingthe absolute intensity value at the photodetector to find the opticalcavity lengths a ratio of the signals at the two distinct wavelengthsmay be used. This reduces errors for instance, those due to insertionloss changes during the life of the sensor, whether due to degradationor connector variability, as the light from each laser will be subjectto similar losses but the ratio should remain unaffected by theselosses. The photodetector should be able to discriminate between eachlaser wavelength in order to measure the ratio of signals. The use ofdual lasers may require the frustration of unwanted cavities, by forinstance, deliberately ‘wedging’ optical components to avoid unwantedinterference fringes from arising due to parasitic optical cavities.

In a further alternative embodiment a single laser and a SLD having acoherence length shorter than the shortest optical cavity may be used.The SLD may therefore provide a background reflection signal, whichmeasures the return loss of the sensor independent of the sensor cavitylength. This measured return loss may then be used to compensate forlosses encountered by the laser light.

A further alternative interrogator may be similar to the dual laserapproach but instead use dual SLDs each providing a different centralwavelength. The coherence length of each SLD may be carefully chosen ortuned to particular optical cavities and exclude others, as describedabove. For instance, for either or both SLDs particular coherencelengths may be chosen, such as for instance, d1<coherence length<d3, sothat the interferometer responds only, to pressure, i.e. the d1 cavitylength changes as the membrane 410 is deflected. Again, a ratio ofdetected signals may be used for error compensation, as described above.

Similarly, as an alternative to the one laser and one SLD light sourcethe interrogator may have two SLDs with one of the two SLDs having sucha short coherence length that it will not provide interference fringesfor any optical cavity present (with the other SLD suitable to generatefringes). However, this short coherence length SLD may still besensitive to losses due to misalignment of the sensor and thereforeprovide an internal calibration for the detected signal intensity. Inother words in this configuration the second SLD may provide anormalisation signal.

As a further alternative interrogator a single SLD with a broaderbandwidth may be used with the resultant light resolved by aspectrometer and computer.

FIG. 5 shows a schematic diagram of an alternative interrogation systemsuitable to interrogate the optical sensor 20. Various interrogationmethods may be used depending on the information required from theoptical sensor. Each optical cavity, d1, d2, d3, in the optical sensormay yield different information. Deflection of the membrane 410 due topressure variations may change the length of d1, for instance. Thermalexpansion may change d2 and d3 and so indicate temperature. Any or allof these optical cavities may be interrogated. This alternativeinterrogator may comprise a Mach-Zehnder interferometer 300 illuminatedby a SLD 320 (having a coherence length less than each optical cavity,d1, d2 or d3, to be interrogated) via a single mode fibre 370 pigtailedto the SLD. It should be noted that such an SLD would not be able todiscriminate any of the optical cavities, d1, d2 or d3 used alone butrelies instead on the Mach-Zehnder interferometer. However, theMach-Zehnder interferometer effectively restores coherence in order togenerate inference fringes for one or more particular optical cavities.Within the Mach-Zehnder interferometer 300 there is at least one phasemodulator 310 and preferably two. Fibre 70 is coupled to the side of theMach-Zehnder interferometer 300 opposite the SLD 320. An interrogatorchip comprising the Mach-Zehnder interferometer 300 may be constructedon any suitable integrated optics platform, such as silicon oninsulator, for instance.

Light, spectrally modified by the Mach-Zehnder and control electronics(not shown), adjusts the signal to correspond with the cavity length ofinterest in the optical sensor. The light is then fed through a 3 dBcoupler 350 to the sensor head, which is shown schematically here assensor element 20, and a collimation lens 60. Half of the return lightfrom the 3 dB coupler 350 is channeled back to a photodiode 330 andthence to the detection electronics. Such an interrogator may besuitable for interrogating any or all of the optical cavities, d1, d2and/or d3 to obtain pressure and/or temperature information. Forinstance, the expansion or elongation of more than one optical cavitymay be used to reduce inaccuracies in temperature determination.

FIG. 5 a shows a schematic diagram of a further embodiment similar tothe example shown in FIG. 5. However, in this further embodiment asecond photodiode 335 is tapped into the single mode fibre 370 using afurther coupler 375. The tap may draw off around 5% of the light (orother small proportion). Therefore, this second photodiode 335 providesa signal proportional to the emission of the SLD 320. This signal may beused to cancel out any variation in the signal obtained from the firstphotodiode 330 due to variations or noise in the SLD output. Forinstance, the signal from the first photodiode 330 may be divided by thesignal from the second photodiode 370 to provide a corrected signal.This corrected signal may provide an improvement in performance.

FIG. 5 b shows a schematic diagram of a dual SLD example implementation.A DC power supply 600 drives the system from an AC mains supply.However, power may be obtained from other sources especially when thedevice is used to monitor engines in vehicles (e.g. portable powersources). A first SLD 320 and a second SLD 325 are supplied with powerusing filtered drive circuits 610, 620. In this embodiment each SLDsupplies a different centre wavelength. The light outputs from each SLD320, 325 are combined in a beam combiner 630. A small proportion (˜5%)of the combined light is tapped off from a transmitter fibre. The twowavelengths are separated using a course wavelength division multiplex(CWDM) demultiplexer (transmit demultiplexer 640) and the wavelengthseparated light is provided to two photodiodes 335, 335′ in order toprovide error correction signals A and B to cancel out any noise and/orintensity output variation generated by each SLD, respectively. Thewavelength bandwidth for each SLD (or other dual or multiple lightsources) may be non-overlapping.

The remaining dual wavelength signal (˜95%) is directed to the sensorhead 10 (via the 3 dB coupler 350), which contains the sensor element 20and operates as described previously. Reflected light is collected bythe 3 dB coupler 350 and demultiplexed in a CWDM receiver demultiplexer660. The resultant light is therefore again separated by wavelength andsampled by receiver photodiodes 330, 330′ providing receiver signals Cand D corresponding to each wavelength. The received signals C, D areanalysed using analysis electronics 650 or a suitably configuredcomputer system. The analysis electronics divides the signals accordingto the following scheme: C/A and D/B. This division step reducesvariations due to connector losses or losses in the sensor head 10, assuch losses will be similar or the same for both wavelengths. Thissignal processing also reduces amplitude fluctuations due to each SLD.To determine where on an interference fringe the output lies, (C/A) isthen divided by (D/B) to provide a corrected signal.

Alternatively, a single SLD may be filtered to provide two outputs orwavelengths. Such optical filtering is shown schematically in FIG. 13.For example, the wavelength bandwidth of SLD 320 may be approximately 30nm (x). A notch filter 710 may selectively reflect a narrow wavelengthband of light to form a first optical signal 720 having a bandwidth ofaround 13 nm (y). The remaining light may pass through the notch filter710 to provide a second optical signal 730. The two optical signals maybe used as if they originated from two sources or SLDs, as describedabove.

Alternatively, other optical filters may be used so provide two narrowbands. The band width for each narrow band may be around 15 nm, forinstance. These narrow bands may be centred on any convenient wavelengthincluding 1510 and 1550 nm, for instance.

Other bandwidths and wavelengths may be used as appropriate.

In an alternative embodiment the Mach-Zehnder interferometer 300 can beplaced between the sensor and the detector with the same effect.

FIG. 5 c shows a schematic diagram of a three-interferometerarrangement. Each one of the three interferometers 301, 302, 303 is aMach-Zehnder interferometer tuned or nominally matched to a differentone of the three cavities d1, d2 or d3 of interest. A three-way splitter350′ divides the signal for each interferometer. Three photo diodes 330,335, 337 are shown coupled to each of the three Mach-Zehnderinterferometers 301, 302, 303. Alternatively, multiplexing may be usedso that all three Mach-Zehnder interferometers 301, 302, 303 may becoupled to a single photodiode. FIG. 5 c shows each Mach-Zehnderinterferometer 301, 302, 303 between the sensor element 20 and the threephotodiodes 330, 335, 337. Alternatively, the Mach-Zehnderinterferometers 301, 302, 303 may instead be between the light source320 (e.g. SLD) and the sensor element 20.

More than three interferometers may be used in a similar arrangement tothat shown in FIG. 5 c, especially to interrogate more than threecavities. Other arrangements of interferometers may be used, forinstance, where one (or more) of a plurality of interferometersinterrogates multiple cavities whilst the remaining interferometers aredirected to a single cavity.

The method of cavity interrogation is described in “Phase-nullingfibre-optic gyro”, Cahill and Udd, Opt. Lett. Vol. 4, pp 93.

A phase shift may be applied to the Mach-Zehnder interferometer 300 suchthat its spectral transmittance exactly matches the spectrum due to thesensor element's 20 reflected signal. A small dither signal (forinstance, a triangle wave) may be applied to the phase shifter 310, thenbecause of the symmetry of the transfer function locally, the resultingsignal may be symmetrical, i.e. the output from the detector at the twodithering positions may be equal so that an error signal, being thedifference between them, may have a magnitude of zero. However, wherethe Mach-Zehnder interferometer 300 is not initially at a null pointthen the signal at the detector generated at the two extremes of thedither signal may be equal to each other and their difference may giverise to a non-zero error signal. This error signal may be suitablyprocessed to be used to instruct a change in the Mach-Zehnder off-setphase to minimise the error signal. The dither signal may be up toseveral MHz, for instance. Therefore, changes in the dimension d1 of theoptical cavity 200 occurring up to several 100 kHz or about 1 MHz may beaccommodated and detected. In this way, acoustic measurements can bemade.

A further alternative embodiment may use white light interferometry witha two-beam interferometer as described in US 2006/0061768. A distributedsensor such as for example, a CCD may be used although this may not beparticularly suitable for acoustic measurements.

The phase modulator 210 may be a PIN diode phase modulator as describedin WO 99/24867, WO 99/60341 and US 2005/0157305. Such a phase modulatormay have associated control electronics capable of establishing the pathlength required to match any or all in sequence of the cavitiespresented by the sensor at a speed and resolution adequate for highdynamic range acoustic measurements.

Alternatively, a number of Mach-Zehnder interferometers may receive thesignal from the sensor element, but each one may be optimised for oneparticular cavity from the sensor, and feed to its own photodetector andassociated amplifier.

FIG. 6 shows a graph of received light intensity versus the optical pathdifference induced in the phase modulator 310 for a sapphire sensorelement 20 made up of a sheet of sapphire 150 μm (d3) thick with avacuum cavity (d1) of 40 μm and final sapphire layer (which forms amembrane that responds to pressure) of 100 μm (d2).

The graph of FIG. 6 shows a minimum in response at 352 μm and 960 μm anda maximum at 432 μm. This is due to the phase change at reflection fromgoing to higher index material to a lower index. This corresponds to thepath length of the first piece of sapphire 100 μm thick (multiplied byrefractive index of sapphire 1.76 and by 2 because the path is onlytraversed once in the Mach Zehnder but twice in the sensor). If 2 times40 μm is added to this value one gets 432μ, and if 150μ multiplied bytwo and by 1.76 is added to 432μ one gets 960μ. The detection algorithmcan either scan either side of these three values and therefore use oneMach-Zehnder interferometer to ‘look’ for three different minima, or thelight returning from the sensor may be fed into three Mach-Zehnderinterferometers that each look around one of the cavity lengths (d1, d2or d3) and minimise the signal into their own photodetector. Note thatin this case it would not be appropriate to have a Mach-Zehnderinterferometer between the light source and the sensor element 20. If,for example a material with a significant electro optic effect was usedsuch as lithium niobate then the phase change could be achieved usingthe electro optic effect.

The graph of FIG. 6 shows other signals due to parasitic cavities.However, for clarity, discussion of these parasitic cavities is omitted.

As will be appreciated by the skilled person, details of the aboveembodiment may be varied without departing from the scope of the presentinvention, as defined by the appended claims.

For example, the alignment technique may be used with an alignmentmechanism other than the ball joint device described above.

Other suitable optical fibres for use as the waveguide, include photonicband gap fibres or LEAF fibres, for instance.

Lens 60 may be a GRIN lens, silica aspheric or spherical convex or otherlens suitable for use at 600-700° C.

Optical surfaces not providing an optical cavity surface may beanti-reflection coated by for instance, “moth eye” or other suitablecoating techniques.

The ball joint may be fixed in place after alignment by e-beam weldingor other technique that avoids distorting the housing body due to thegeneration of excessive heat.

More than one lens may be included to collimate or focus the beamemitted from the optical fibre 70. This allows larger diameter beamsthat improve efficiency of illumination and collection.

Other materials may be deposited on to the membrane 410 to increase thethermal response of the sensor. For instance, SiC or Si, suitablypassivated with a material such as silicon nitride, may be used. Suchmaterials provide an amplified thermal effect which allows highertemperature sensitivity further allowing less material to be usedreducing the time constant for thermal detection and monitoring.

The geometry of the sensor may be changed to allow the beam emitted fromthe optical fibre 70 to travel in the plane of the membrane 410. Thismay result in an optical path of up to several mm to be achieved for amembrane thickness d2 of a few μm. This provides a faster temperaturesensor due to a reduction in the required mass.

The cavities of the optical element 20 may be interrogated with a duallaser, a laser and SLD to allow discrimination between long and shortcavities or with a slave interferometer with broadband light.

The components of the optical sensor may be assembled at a suitablyelevated temperature to ensure compression over the working temperaturerange.

In an alternative embodiment more than one fibre may be bonded to rearof the collimator lens to improve strength and durability, one opticalfibre carries light and the others are for mechanical purposes.

To improve mechanical strength of the lens 60 to optical fibre 70 jointfurther glass encapsulation around this joint may be used.

Other thermal insulation may be used to maintain the optical fibre 70cooler than the dielectric body, including partial or full vacuum or gasfilling.

In an alternative embodiment the alignment mechanism for the bondedoptical fibre 70 and lens 60 assembly may include a cylindrical springclip arrangement as shown in FIG. 7. A cylindrical spring clip 400retains the lens 60 by supporting the lens 60 around the circumferenceof the lens 60. This reduces the effect of thermal expansion on theaxial alignment of the lens 60.

FIG. 8 shows a perspective view of the spring clip 400 with the lens inplace. A plurality of spring fingers 410 grip and apply a force to thecircumference of the lens 60.

FIG. 9A shows a fibre-lens assembly according to a further embodiment ofthe present invention. A cylindrical substrate 510 having two parallelplanar surfaces is used to form the lens 60′. A lens surface 500 isformed as a curve by etching or micro-machining one planar surface ofthe substrate 510 so that a curved surface is formed surrounded by aplanar surface. The substrate 510 may be made from sapphire or glass,for instance. FIG. 9C shows an expanded view of the lens surface 500.The lens surface 500 may be spherical or aspheric. It may also be apositive or negative lens. In one embodiment the radius 520 of the lenssurface 500 is 0.5 mm but may be smaller. The radius of curvature of thelens surface may be 0.25 mm, for instance. The dimensions depend on therequired lens-optical sensor separation. The circumferential portion ofthe cylindrical substrate 510 provides a support surface for the lens60′ that is normal to the optical axis of the lens 60′.

The planar surface surrounding the lens surface 500 may then abut acorresponding surface in the end cap 90 or 90′ or other suitable portionof the mount 40 or 40′. As the surfaces are relatively large comparedwith the lens surface 500 alignment of the lens may be simplified. Theplanar surface surrounding the lens 60 may be urged against the end cap90 or 90′ or mount 40 or 40′ by the spring fingers 410 of the springclip 400 applying an axial force on the lens 60, e.g. by the springfingers 410 bending to some extent at the point that they meet the bodyof the spring clip 400. Further accuracy in alignment of the lens 60 maybe achieved by the abutting surface of the cap 90 or 90′ or mount 40 or40′ having a bore or chamfer corresponding to the extent of the lenssurface 500 such that the lens surface 500 is received by the bore orchamfer.

A planar lens may improve the alignment of the device as it may seatmore accurately within the housing (i.e. relative to the optical axis).The fibre 70 and lens arrangement may then be aligned, as describedabove, before being welded in place by a CO₂ laser, for instance. Thispassive alignment technique may be extended to other lens typesincluding rod lenses, for instance.

FIG. 9B shows a cross-sectional view along line C-C of FIG. 9A. Theopposite planar surface of the substrate 530 (i.e. opposite the etchedsurface containing the curved lens surface 500) is bonded to the opticalfibre 70, as described above with reference to FIGS. 1 and 3, to thenormal of the planar surface 530. Thermal fusion bonding or othersuitable bonding may be used. The lens surface 500 as well as theoptical fibre bonding point may be centred on each planar surface of thesubstrate 530 but should at least be opposite each other and collinear.

The cylindrical substrate 510 may be 3 mm in diameter 540 and 1.5 mmthick 550, for example.

FIG. 10 shows a perspective view of the cylindrical substrate 510 andfibre 70 of FIGS. 9A-C. FIG. 11 shows a magnified view of the lenssurface 500 within the etched or micro-machined region of the substrate510. This embodiment allows easier handling of the lens 60′ duringmanufacture and alignment. Although a cylindrical substrate 510 is shownother profile shapes such as square, rectangular and hexagonal, forinstance, may be used.

The substrate 510 may be supported by the cylindrical spring clip 400 asdescribed with reference to FIGS. 7 and 8 or other suitable support oralignment mechanism. FIG. 12 shows the lens 60′ and fibre 70 arrangementheld by a suitable spring clip 400 similar to that described withreference to FIG. 8.

Alternatively, the lens may be a fresnel lens or an array of smallerlenses (e.g. 20-50). These types of lenses may be made more easily usinglithographic and/or wafer scale dry etching techniques where scalesdeeper than 20 μm are difficult to fabricate.

FIGS. 14 a-d show four views of an optical sensor according to a furtherembodiment. FIG. 14 a is an end view, FIG. 14 b is a side view and FIG.14 c is a cross-sectional view along the line A-A of FIG. 14 b. FIG. 14d shows a perspective view of the optical sensor. In this embodiment aninner support tube 30 supports a sensor element 20 in a similar mannerto that described with reference to FIG. 1 (although with a hollow tuberather than solid). Similarly, a lens 60 has been welded to a fibre 70to provide a collimated beam for illuminating and collecting light fromthe sensor element 20.

However, in this embodiment the inner support tube or spacer 30 isitself spaced apart from the housing or case 40 to improve mechanicaland/or temperature isolation. In this case, such a spacing is achievedby a spacing projection in the form of an annular projection 800provided around the inner support tube (spacer) 30. This provides asingle or isolated fixing point for forming a connection between theoptical components and the outer casing or housing 40. In other words,the optical components, including the sensor element 20, lens 60 andfibre 70 may be substantially mechanically and/or thermally isolatedfrom the outer casing 40, whilst maintaining support and environmentalprotection. The inner support tube or spacer 30 is substantiallyseparated from the outer casing 40 by a gap, which may be filled withgas or other fluid or evacuated to provide additional thermal shockprotection and isolation.

This embodiment improves both thermal and mechanical isolation of theoptical components. The heat path from the environment external to theouter casing 40 (which may be at elevated temperatures) to these opticalcomponents is therefore limited to this single point of contact.Alternatively, the annulus 800 may be broken to limit the contact areabetween the optical components and the outer casing 40. In this casemore than one point of contact may be made (i.e. by several lugs orshims) yet still improving thermal and mechanical isolation.

The spacer 30 does not experience significant mechanical loads butprovides a thermal shunt. The annulus 800 provides asymmetrical fixingpoint so that any resonances may be well controlled and predicted. Theresonant frequency of the optical components within the inner supporttube 30 may be configured out of the range of pressure frequencies orsound that the device is designed to detect. For instance, a resonantfrequency above about 9 kHz may be suitable and can be adjusted byvarying the length or mass of the inner tube or spacer 30. Tapering ofthe inner tube 30 may also damp out certain frequencies or resonances,if necessary.

The point of contact may be set back from the sensor element 20 end ofthe device, i.e. away from the hot end in use. This may further limitthe heat conduction to any optical elements, including the fibre/lensjoint. The hot end may act as a thermal shunt to keep the inner opticalassembly more tied to the back end (cooler) temperature.

This embodiment may improve resilience to distortion for instance, whenthe device is knocked or fixed (screwed) into place.

In this further embodiment the fibre may be actively aligned beforebeing fixed in position, as described previously.

The invention claimed is:
 1. An optical sensor comprising: an opticalcavity defined by a dielectric body and responsive to one or morephysical environmental conditions; and a waveguide having a terminal endattached to a lens and spaced apart from the optical cavity such thatlight is optically coupled from the terminal end of the waveguide to theoptical cavity through the lens, wherein the distance between theterminal end of the waveguide and the optical cavity is sufficient suchthat, in use, the waveguide is maintained at a first temperature thatwould not damage the optical or mechanical properties of the waveguideand the dielectric body is maintained at a second temperature higherthan the first temperature and that would be sufficient to damage theoptical or mechanical properties of the waveguide; the optical sensorfurther comprising an optical alignment mechanism for providingalignment between the waveguide and the optical cavity, the opticalalignment mechanism comprising a ball joint having a ball arranged tosupport the lens and a socket arranged to adjustably support the ball.2. The optical sensor of claim 1, wherein the distance is greater than10 mm.
 3. The optical sensor of claim 1, wherein the ball is fixed inposition relative to the socket by a fixture.
 4. The optical sensor ofclaim 3, wherein the fixture is a weld.
 5. The optical sensor of claim 1further comprising a housing having a proximal end and a distal end andarranged to support the dielectric body at the proximal end and tosupport the waveguide at the distal end.
 6. The optical sensor of claim5, wherein the ball joint is arranged within the housing.
 7. The opticalsensor according to claim 1 further comprising a collimator forcollimating the light emitted from the waveguide.
 8. The optical sensorof claim 7, wherein the collimator is a beam expander.
 9. The opticalsensor of claim 7, wherein the collimator comprises the lens.
 10. Theoptical sensor of claim 9, wherein the attachment between the terminalend of the waveguide and the lens is a fusion bond.
 11. The opticalsensor of claim 9, wherein the lens comprises a first surface includinga curved portion surrounded by a planar portion.
 12. The optical sensorof claim 11, wherein the lens further comprises a planar second surfaceparallel with the planar portion of the first surface.
 13. The opticalsensor of claim 11, wherein the curved portion is formed bymicro-machining.
 14. The optical sensor of claim 11, wherein the curvedportion is formed by etching.
 15. The optical sensor according to claim9, further comprising a lens mount.
 16. The optical sensor of claim 15,wherein the lens mount comprises a plurality of spring fingers forsupporting the lens.
 17. The optical sensor of claim 16, wherein theplurality of spring fingers apply axial force to the lens.
 18. Theoptical sensor according to claim 9, wherein the lens is formed fromsapphire or silica.
 19. The optical sensor according to claim 9, whereinthe lens is moulded.
 20. The optical sensor according to claim 1,wherein the optical cavity in the dielectric body is defined at one endby a membrane deflectable in response to changes in external pressure.21. The optical sensor of claim 20, wherein the optical cavity isdefined between the membrane and a dielectric slab and wherein a secondoptical cavity is defined by the two parallel faces of the dielectricslab such that light is optically coupled from the terminal end of thewaveguide to the optical cavity; and said second optical cavity isresponsive to changes in temperature due to thermal expansion orcontraction of the dielectric slab causing a variation in the separationof the two parallel faces.